Conformational isomers of the [(5‐methyl‐2‐pyridinio)aminomethylene]diphosphonate dianion and [(5‐methyl‐2‐pyridyl)aminomethylene]diphosphonate trianion in salts with 4‐aminopyridine and ammonia

The crystal structures of two salts, products of the reactions between [(5‐methyl‐2‐pyridyl)aminomethylene]bis(phosphonic acid) and 4‐aminopyridine or ammonia, namely bis(4‐aminopyridinium) hydrogen [(5‐methyl‐2‐pyridinio)aminomethylene]diphosphonate 2.4‐hydrate, 2C₅H₇N₂⁺·C₇H₁₀N₂O₆P₂²⁻·2.4H₂O, (I), and triammonium hydrogen [(5‐methyl‐2‐pyridyl)aminomethylene]diphosphonate monohydrate, 3NH₄⁺·C₇H₉N₂O₆P₂³⁻·H₂O, (II), have been determined. In (I), the Z configuration of the ring N—C and amino N—H bonds of the bisphosphonate dianion with respect to the C_ring—N_amino bond is consistent with that of the parent zwitterion. Removing the H atom from the pyridyl N atom results in the opposite E configuration of the bisphosphonate trianion in (II). Compound (I) exhibits a three‐dimensional hydrogen‐bonded network, in which 4‐aminopyridinium cations and water molecules are joined to ribbons composed of anionic dimers linked by O—H...O and N—H...O hydrogen bonds. The supramolecular motif resulting from a combination of these three interactions is a common phenomenon in crystals of all of the Z‐isomeric zwitterions of 4‐ and 5‐substituted (2‐pyridylaminomethylene)bis(phosphonic acid)s studied to date. In (II), ammonium cations and water molecules are linked to chains of trianions, resulting in the formation of double layers.